The present invention is generally related to electromechanical filters and more specifically, is related to electromechanical resonators used as input and output transducers in multiresonator bandpass mechanical filters. The present invention is even more specifically related to instances where the electromechanical resonators need to operate in a torsional mode.
In the prior art, electromechanical resonators are used to convert electrical signals into numerous modes of vibration including flexure, torsion and extension, and to convert mechanical energy into electrical energy. In mechanical filters, an input electromechanical resonator is acoustically coupled to mechanical resonators in the interior of a filter, and which, in turn, are coupled to an output electromechanical resonator to form a bandpass filter. Such mechanical bandpass filters formed by the combination of electromechanical resonators and one or more mechanical resonators have a wide variety of uses in conventional communication systems including telephone, radio and avionics applications.
In such prior art systems, advances in technology demand miniaturization of various electronic components and systems. In an effort to miniaturize, some prior art attempts have employed torsional interior resonators in connection with mechanical bandpass filters in the 400-600 kHz range. To meet the requirements for a rugged construction second-mode torsional interior resonators are preferred, because second-mode torsional resonators possess two nodes to which supports can be attached. In such mechanical filters using torsional interior resonators, the selection of a torsional electromechanical resonator as the electromechanical resonator for the input and output of the filter is natural.
Typically, torsional electromechanical resonators include the use of resonator rods in conjunction with a commercially available ceramic transducer. In some instances, the resonator support pin and lead wire arrangement is insufficient to meet the rugged construction requirements and has insufficient versatility to produce sufficient electromechanical coupling required for many bandpass filters. The lead wire attachment to the ceramic disc or rod also increases costs and causes undesirable frequency variations.
In still other instances, second-mode torsional electromechanical resonators have been constructed using twin lead supports, for conducting the electrical signals into the resonator in lieu of the lead wire attachment to the ceramic disc. While this design provides a more rugged construction for use in mechanical filters, there is still insufficient electromechanical coupling for many mechanical bandpass filter configurations. Such limitations result from the need for maximum coupling, which can only be achieved by locating the ceramic transducer at one of the locations of maximum stress, which are, in fact, the nodes, which are the positions used to physically support the electromechanical resonator. Furthermore, spurious mode frequencies of the second-mode electromechanical resonator are close to those of the second-mode torsional interior resonators because of similar lengths, thereby allowing spurious frequencies to be passed by the filter. Each of these drawbacks limit use to which the electromechanical resonators may be applied in forming mechanical bandpass filters.
Accordingly, the present invention has been developed to provide an improved torsional electromechanical resonator which may be used to produce improved construction, coupling and spurious rejection in a miniaturized mechanical bandpass filter.